Controlling Kinetic Pathways in Demixing Microgel–Micelle Mixtures

We investigate the temperature-dependent phase behavior of mixtures of poly(N-isopropylacrylamide) (pNIPAM) microgel colloids and a triblock copolymer (PEO–PPO–PEO) surfactant. Usually, gelation in these systems results from an increase in temperature. Here we investigate the role of the heating rate, and surprisingly, we find that this causes the mechanism of aggregation to change from one which is driven by depletion of the microgels by the micelles at low temperatures to the association of the two species at high temperatures. We thus reveal two competing mechanisms for attractions between the microgel particles which can be controlled by changing the heating rate. We use this heating-rate-dependent response of the system to access multiple structures for the same system composition. Samples were found to demix into phases rich and poor in microgel particles at temperatures below 33 °C, under conditions where the microgels particles are partially swollen. Under rapid heating full demixing is bypassed, and gel networks are formed instead. The temperature history of the sample, therefore, allows for kinetic selection between different final structures, which may be metastable.

: Figure highlighting the editing process used to highlight the regions rich in polymer in images taken using the DIC microscope. DIC images are converted to grey scale, then converted to glow, then the images inverted so that the polymer rich regions appear bright and the polymer poor regions appear dark.The scale bar is 40 µm. Figure S2: Hydrodynamic diameter of the pNIPAM microgels as a function of temperature. Figure S1 highlights how differential interference contrast (DIC) images were edited in this work. The images were converted to grey scale, then converted to the glow colour palette, then the image inverted. The brightness of the image was then adjusted. This results in the polymer rich regions appearing bright and the polymer poor regions appearing dark. Figure S2 contains the deswelling data for the pNIPAM microgels used in this study, determined using dynamic light scattering. Figure S3 highlights the phase behaviour of pNIPAM microgel in the presence of triblockcopolymers as a function of pNIPAM volume fraction. The volume fraction of the microgels S2 Figure S3: Figure highlighting the phase behaviour pNIPAM microgels in the presence of triblock-copolymer as a function of volume fraction. The pink data point indicate samples that form gels, the blue data points indicate the samples that phase separate. Each data point is at a different temperature as the volume fraction of the microgels is temperature dependent. Figure S4: Images of droplets coalescing over time. The concentration of the sample is 4 wt% pNIPAM and 8 wt% triblock-copolymer. The scale bar is 40 µm.
is temperature dependant, as the microgels collapse at increased temperature. Therefore, all the data points are at taken at different temperatures. Each data point is the lowest temperature where aggregates were observed, represented as volume fraction rather than temperature as in Fig 4 in the main text. At lower volume fractions of pNIPAM gels form, indicated in pink. At higher volumes fractions of pNIPAM, phase separation occurs, indicated in blue. The volume fraction has been estimated from the volume fraction of water inside the microgels and the diameter and various temperatures. ? Figure S4 highlights the evolution of the droplet phase over time. Droplets begin to form upon heating, then when held at 33 • C, the droplets continue to grow and eventually merge.
There is a general increase of droplet size with time, tending to a plateau value at late time.
It can be seen that droplets grow to larger sizes when held at 33 • C (a), rather than heated to 35 • C and held at that temperature. It was also observed that a power law can be used to describe the data set, where samples heated to 33 • C scale with gradient µ = 0.46, whereas samples heated to 35 • C scale with µ = 0.36. Figure S6 illustrates how the size of the droplets increases with time. Figure S7 contains the rheological profile for 5 wt% pNIPAM, 3 wt% triblock-copolymer.
In the samples that undergo syneresis, the sample pulls away from the plate resulting in the rheological profile looking similar to pure water rather than a hydrogel. It is clear that the S4 Figure S6: a-d) Optical micrographs of 3 wt% pNIPAM, 9 wt% triblock-copolymer heated to 33 • C at a rate of 0.1 • C/min and held at that temperature. The time was started from the first indication of gelation. The scale bar is 10 µm .
full profile of the pNIPAM samples cannot be accessed therefore this technique is not an accurate method to compare the structures produced in this work.  Table S1 includes the volume fractions calculated for the pNIPAM microgels at different wt%.
Videos are also included in the supplementary information to show the formation of the different phases observed in this work. Figure S7: Temperature ramp rheology profile for a sample of pNIPAM (5 wt%) and triblockcopolymer (3 wt%). The strain used was 1% and the frequency was 1 Hz